Chemical Engineering May 2012 - 61

FIGURE 3. This single-impeller
centrifugal
compressor with axial
(horizontal) inlet and
vertical discharge, is
an example of a centrifugal
compressor for
a process booster application
FIGURE
2. This cutaway diagram shows
the internals of a horizontally split centrifugal
compressor. With this horizontally
split casing, major maintenance is simpler
with downward piping connections,
because there is no need to disturb the
pipe-work when opening the casing
forged, are generally specified for medium-
or high-pressure CPI machines.
For flammable or toxic process gases,
a suitable steel grade is the minimum
requirement for the compressor casings.
Generally, casings should be heat
treated regardless of thickness.
Many complex CPI applications cannot
be accommodated by a single casing
compressor. A good example is a
high-pressure ratio service (say more
than eight). Multiple compressor casings
are commonly employed. A popular
configuration is the tandem-driven
series arrangement using a common
driver. A gear unit may be included in
the compressor train, either between
casings or between the driver and the
compressor casings.
Compressor component design
Shafts should be made of one piece,
heat-treated forged low-alloy-steel,
suitably ground. It should be forged
as close as possible to the final dimensions.
Forged, low-alloy shafts are
standard shafts for process centrifugal
compressors. Only machines handling
highly aggressive gases may have corrosion
resistance shafts. Shaft sleeves
are frequently fitted so that sealing elements
do not operate directly against
the shaft.
Two types of impellers are commonly
used: a closed impeller (consisting
of a hub, blades and a cover) and
a semi-open impeller (consisting of a
hub and blades). The semi-open impeller
is most often called an open impeller
for simplicity.
FIGURE 4. The steam turbine driver shown here is a horizontally split machine with
many axial-fl ow stages. Steam turbine drivers offer speed match (direct drive, without
gear unit) and variable-speed drive operation
Impellers are fabricated by weld,
braze, mill, electro-eroded or cast
(most often fabricated using forged
components). Plating and other methods
of metal buildup on an impeller
are generally unacceptable. Impellers
need heat treatment (stress relief).
Semi-open impellers offer a high flow
coefficient. They are usually used for
gases with a molecular weight below
40. Semi-open impellers introduce
more vibration and dynamic issues
compared to the closed-type.
Closed impellers have a smaller axial
length than the semi-open type, and
are easier to fabricate. These impellers
are more popular in modern centrifugal
compressors (except integrally
geared and overhung machines that
use semi-open impellers). Impellers
are shrunk onto the shaft, either hot or
hydraulically. For shrinking on impellers,
great care should be taken. When
cooling down, the impellers should not
pull a bend into a shaft. Some compressors
use polygon-fitted impellers.
It is economically desirable to use
the smallest possible compressor. The
higher the flow coefficient, the larger
the suction rate for a given impeller
diameter. For a multi-stage compressor,
the first stage impeller is chosen
to have the maximum flow coefficient.
Sometimes a semi-open impeller is
used for the first stage; however this
configuration is rarely specified today.
The flow coefficient of the subsequent
impellers decreases as inlet volume
decreases, if the shaft speed is constant.
In multi-casing trains, the first
casing, because it has the highest suction
volume, dictates the train speed.
This could lead to increasingly sub-optimum
designs as the suction volume
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